MICROBIALITES IN THE COCKBURN TOWN MEMBER FOSSIL REEFS, SAN SALVADOR, BAHAMAS: A NEW LOOK AT A CLASSIC SANGAMONIAN DEPOSIT

Since the pioneering study by Curran and White (1984), the Sangamonian coral-algal reefs of the Cockburn Town Member (Grotto Beach Formation, San Salvador, Bahamas) have been the focal point of numerous paleontologic, stratigraphic and geochronologic studies for nearly three decades. The fossil reefs remain pivotal in the ongoing discussion regarding the nature of the MIS 5e sea-level highstand. Reef-building metazoans and associated invertebrates have been well documented, yet by comparison, the occurrence of microbialites in the Cockburn Town reefs is poorly known. Cockburn Town Member microbialites were first identified by Jones and Hunter (1991) and were briefly described in the context of a prevalent biotic transition in Caribbean Pleistocene reefs. However, the character, development and distribution of microbialites within Cockburn Town Member reefs have yet to be detailed.

Preliminary examination of reef facies at the Cockburn Town Quarry and Sue Point localities reveals numerous examples of thick, laminated carbonate coatings on in situ and fragmented branching reef corals (Acropora cervicornis, Porites porites and Acropora palmata), particularly near the top of the reef and in the deposits immediately below a widespread erosional discontinuity associated with temporary cessation of reef growth. In the field, these thick laminar coatings are commonly attributed to encrustations of corallinacean algae (rhodolite). Petrography reveals that these coatings begin with a thin interval of rhodolite, but quickly transition to laminated microbialite with only minor corallinacean and foraminferal contributions. These microbialites form cm-scale buildups of micritic to sandy carbonate with a range of forms and internal fabrics described in this study. Locally, the microbialites expand from coral surfaces to form independent buildups within adjacent sediments.

Numerous studies of post-mortem encrustation on modern reef corals suggest that the rhodolite layers present in the Cockburn Town Member could accumulate over a short time frame (years to a few decades) in response to normal biotic or physical disturbances in the reef environment. However, the thick microbialites present in two intervals of the Cockburn Town reefs would likely require a longer term environmental change.